In the purification of polymerizable vinyl monomers, there is a tendency for polymer to form on the internal surfaces of the distillation column and associated heat exchange vessels and piping.
Eventually, polymer buildup completely plugs the tower or decreases its efficiency significantly. Present methods available for cleaning distillation towers involve removing packing and/or hydroblasting the packing and internal walls in-place. Time lost from production is costly and labor costs are considerable. For some polymers, the complete replacement of column internals is the only way to remove them.
In the purification of acrylic acid, the polymer build-up is particularly difficult to remove. There may also be copolymers formed which include aldehydes. The crosslinked polymers and copolymers are especially insoluble. The homopolymer of acrylic acid which forms has a variety of physical forms, some of which are highly insoluble. The polymer may form as a thin film around the walls of the column where condensation in the absence of inhibitor occurs or may form as small masses throughout the packing in the shape and size of "popcorn" . The polymer may also be clear and gel-like or a hard, white, opaque mass. Color is generally clear or white. However, in areas near the bottom of the column, the polymer may be brown to black in color where decomposition products have concentrated.
Although the low molecular weight polymer of acrylic acid is readily water soluble, under harsh conditions, i.e., too high a temperature and/or low amounts of inhibitor, the insoluble polymers will form. The common organic and inorganic solvents will not dissolve the polymer formed in columns. Thus, it would be very desirable to find a method which would remove such polymers effectively in-place, i.e. without removing any of the column internals, and with a minimum of down-time for the equipment.
It has now been found that certain dialkyl phthalates will dissolve and/or decompose these objectionable polymers.